Practical aspects of internal field NMR spectroscopy application

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Short review on FTS

The first experiments on catalytic hydration of carbon monoxide CO were performed by Sabatier and Senderens in 1902 by methane synthesis from a CO and CO2 mixture with hydrogen. In 1922, Hans Fischer and Franz Tropsch proposed the Synthol process [163] using iron chips as catalyst. Significant progress in FTS has been made in 1923. Longer hydrocarbon (HC) chains [164] were obtained while decreasing the overall pressure to 0.7 MPa. In 1926, the first report on HC synthesis has been published [165].
In 1934, FTS has been certified by Ruhrchemie and was industrially applied within two years. In 1936, the first large-scale reactor has been launched in Braunkohle-Benzin. In 1938, Germany reached ~13,000 barrels per day production of raw product. After the Second World War, Arbeitsgemeinschaft Ruhrchemie und Lurgi (ARGE) established large-scale plants based on fixed bed reactors. Simultaneously, Kellog proposed the concept of circulating catalyst bed. Both the Kellog and ARGE processes have been used by Sasol in the RSA. Sasol 1 plant has been constructed in 1955 in Sasolburg, Sasol 2 and Sasol 3 in Secunda in 1980 and 1982, respectively [166]. The main progress brought by Sasol is summarized in a monograph (monograph chapters [167–175]).
In 1980-s, the interest for FTS has risen again due to the perspective of concomitant gas utilization. The synthetic fuel contains less sulfur and aromatics; therefore it is safer for the environment. The modern interest for FTS consists in biomass conversion as well as natural gas conversion to olefins. In 1993, Shell Bintulu launched a plant producing 12,500 barrels per day. In 2006, Sasol Oryx established a plant with 34,000 barrels per day. Sasol Chevron is currently building its Escarvos “gas-to-liquids” (GTL) plant in Nigeria. Shell and Exxon signed an agreement for building 140,000 and 150,000 barrels per day GTL-FT plants in Qatar. The history of FTS catalyst design has been reviewed by Bartholomew [176].

FTS catalysts

All metals of VIII group have noticeable catalytic activity in monoxide CO hydrogenation towards hydrocarbons (HC) nCO  2nH2 CnH2n  nH2O.
2 2 2 2 (2 1) n n nCO n H C H nH O     .
Iron, nickel, cobalt and ruthenium are the most active metals in FTS process. The average molecular weight decreases in the following sequence: Ru > Fe > Co > Rh > Ni > Ir > Pt > Pd [177]. The most desirable products are C5-C8, especially alpha-olefins that can be easily polymerized to get more plastic materials compared to ones based on paraffin. Thus, only Ru, Fe, Co, and Ni have catalytic performance that can be used in large scale industry. However, Ni in normal FTS conditions produces only methane. Ru is very expensive for large scale usage, and its reserves are not sufficient to be used in FTS. Consequently, it is usually iron and cobalt that are utilized in FTS plants [178].
Co based catalysts are more expensive than Fe ones; however they are more resistant to deactivation. The activity at low conversion rates is comparable, but productivity at higher conversion rates is more significant on Co FTS catalysts. Water created in FTS reaction slows more the reaction rate on Fe catalysts than on Co ones. At relatively low temperatures (473 – 523 K), the probability of chain growth on Co catalysts is 0.94, and on Fe catalysts it is 0.95 [179–181]. The water-gas shift reaction is more significant on Fe than on Co based catalysts. 2 2 2 CO  H OCO  H.
Iron catalysts usually give more olefins. Both metals are sensitive to sulfur content, which easily deactivates them. The upper critical sulfur content in the feed is limited to 0.2 ppm for Fe, and to 0.1 ppm to Co [180–182]. The Co supported on oxides catalysts are in general more resistant to attrition than bulk Fe ones. Fe FTS catalysts produce HC and their oxygenates at different pressures, H2/CO ratios, and different temperatures up to 609 K. Co FTS catalysts work in a very narrow range of temperatures and pressures, and temperature increase leads to selectivity shift mainly to methane. Fe based catalysts in general are more suited for biomass conversion since they can work at lower H2/CO ratio.

Structure of FTS catalysts

The structural study of FTS catalysts is essential for improving their performance (activity and selectivity). Also the knowledge of the catalyst structure is needed to increase the cost efficiency of the catalyst preparation process. The question of the active species in FTS catalysts is also still under consideration.
There are many preparation techniques of Co FTS catalysts such as coprecipitation in solution, mechanochemical activation under inert atmosphere of Co and support and etc. However the most common one is by incipient wetness impregnation (ICP). The impregnated supports are usually γ-Al2O3 [186], but also TiO2 [187], ZrO2, SiO2 [188] or ZSM-5 [70]. The Cobalt source is a water solution of Co2+, mainly as Co nitrate Co(NO3)2·6H2O, chloride CoCl2·6H2O or acetate Co(CH3COOH)2·4H2O. To increase metal content, multistage impregnation is used. Then the catalysts are dried in the air at 353-431 K for up to 16 h, and calcined at 573-673 K for 4-6 h to decompose Co salts into Co3O4 oxide.
Generally, all Co FTS catalysts are promoted by different species, mainly by Noble metal (Ru, Pt, Re) and cations (В, Na, Zr, La). Addition of promoters to catalysts improves properties as activity, selectivity, reduction temperature, etc. The promoter is introduced after impregnation, but before drying and calcination. Standard promoter precursors are ruthenium nitrosyl nitrate Ru(NO)(NO3)3, platinum nitrate tetraammin (II) [Pt(NH3)4](NO3)2, rhenium oxides , boracic acid H3BO3, sodium carbonate Na2CO3, zirconium nitrate Zr(NO3)4, and lanthanum nitrate La(NO3)3 [187; 189].
The Co FTS catalysts on γ-Al2O3 exhibit a two-stage reduction process. The first low temperature stage is Co3O4→CoO reduction, and the second high temperature one is CoO→Co0 reduction. Catalyst promotion by Pt or Ru shifts the first reduction stage to lower temperatures. Re also decreases the second stage temperature. The addition of 1% Re to the catalyst decreases the average Co particle size, but Re does not change the Co0 reduction rate. Re stabilizes the Co particles and as a result smaller Co particles can be obtained [186].


Basics on internal field 59Co NMR

It is essential to start experimental internal field 59Co NMR investigations by looking at bulk Co metal powder since literature data are sometimes contradictory. In this chapter, the internal field NMR is essentially coupled with XRD since these techniques are the only ones that can provide quantitative data on hcp/fcc ratio in Co metal. A method to plot internal field NMR data in universal way is also shown here as well as a classification of resonance lines. The content of this chapter is published in Applied Magnetic Resonance [1].
Despite the fact that a full description of field 59Co NMR lines was done in this paper several important notices can be mentioned. First of all, the particle size of studied particles was 2-5 μm, and the presence of small (< 70 nm) single-domain particles is not expected. At the same time, this sample can be described as bulk material with high number of domains and domain walls. Therefore the demagnetizing field should be averaged to zero. However, we claimed 216.5 MHz line to arise from fcc magnetic domains, which is not the case of Co metal particles of micron size. The residual demagnetizing field (if exists) of magnetic domains in multidomain sample is negligible compared to observed shifts in single-domain particles (several MHz). Therefore, 216.5 line should be attributed to fcc stacking faults.

High temperature strong CoAlO/Co-Al cermets

In this chapter, the internal filed 59Co NMR application to ceramic Co-Al-O materials is shown. The chapter is based on our earliest publication in Journal of Material Science and Engineering A [4]. First of all, the author apologizes for the “Russian” English used in this article. The NMR interpretation in this work has been carried out in accordance with conventional sf1-sf5 defects observation. During the course of this PhD work, this point of view has been re-considered. Nevertheless, the work is of importance since the application of internal field NMR to study cermets (as construction or catalysis related materials) has been reported here for the first time.

Table of contents :

List of acronyms and abbreviations
Academic context
1. Principles of internal field NMR
1.1. Hamiltonian operator in magnetic materials
1.2. Bloch model of internal field NMR.
1.3. Conclusions to Chapter 1:
2. Practical aspects of internal field NMR spectroscopy application
2.1. Co metal
2.2. Cobalt alloys
2.3. Conclusions to Chapter 2:
3. Experimental
3.1. Internal field 59Co NMR.
3.2. Complementary methods
4. Results and publications
4.1. Short review on FTS
4.1.1. Historical perspective
4.1.2. FTS catalysts
4.1.3. Structure of FTS catalysts
4.2. Basics on internal field 59Co NMR
4.2.1. Co metal powder
4.2.2. Physics of Co nanoparticles
4.3. Catalytic and related materials
4.3.1. Supported FTS catalysts
4.3.2. High temperature strong CoAlO/Co-Al cermets
4.3.3. Low-temperature Al2O3/CoAlO/CoAl cermet
4.4. Co/MWCNT hybrids
5. Conclusion
5.1. Summary of results
5.2. Future work
Résumé en français


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